A method of this type is already known from French Patent No. 2 766 123. The method of that French patent is entirely satisfactory, and gives the advantages that stem in particular from the fact that, depending on whether one or more objects are thermoformed from each blank, the use of such blanks makes it possible to avoid or to limit considerably the wastage that exists when a continuous strip of thermoplastic material is used.
Generally, in order to thermoform an object such as a container from a blank, the blank is held in a thermoforming station via its rim, which is not deformed, and a thermoforming step is performed by stretching the central region of the blank to form the body of a container. Thus, the rim of the object, e.g. a container, is constituted by a non-deformed zone of the blank. In general, said zone is flat like the blank is initially, and it is of thickness equal to the initial thickness of the blank, whereas the wall of the body of the container formed from the blank is of a considerably smaller thickness that is a function of the amplitude of the stretching obtained during thermoforming. Thus, the known method of thermoforming from blanks restricts somewhat the geometrical shapes of the rims of the objects obtained by thermoforming.
In addition, depending on the type of object that is to be thermoformed, the initial thickness of the blank is determined so that, after the desired stretching, the body of the object obtained by stretching has a given thickness. For example, the thickness of the wall of the body can be from 10 times smaller than the initial body can be from 10 times smaller than the initial thickness of the blank to 20 times smaller than said initial thickness. However, with the existing method, the thickness of the blank remains unchanged in the region of the rim of the object, and, as a result, in said rim, a quantity of unused material is to be found whereas it is rarely or even never necessary for the rim of such an object to have a thickness as large as the initial thickness of the blank.
An object of the present invention is to improve further the method described by Patent FR 2 766 123 so as to overcome the above-mentioned drawbacks.